Stainless steel for use in seawater applications

ABSTRACT

A duplex ferritic-austenitic stainless steel alloy provided for seawater applications includes, in weight %: C maximum 0.05; Si maximum 0.8; Mn 0.03-4; Cr 28-35; Ni 3-10%, Mo 1.0-4.0; N 0.2-0.6; Cu maximum 1.0; W maximum 2.0; S maximum 0.010; Ce maximum 0.2; and the balance Fe together with normally occurring impurities and additives, wherein the ferritic content is 30-70 volume %, the alloy composition has a PRE-value higher than 42, and the PRE-value is at least 40 in both the ferritic and austenitic phases. A method is also provided which includes: providing a duplex ferritic-austenitic stainless steel alloy with the above-noted composition, forming a component with the alloy, and contacting the component with seawater. In preferred embodiments, the component comprises at least one of tubes, bars, heavy castings, forging, plates, wire or strip.

FIELD OF THE INVENTION

The present invention provides a ferritic-austenitic stainless steelprovided for seawater applications and use of this ferritic-austeniticstainless steel in seawater applications and nearby areas, whereespecially favorable properties for the steel have been attained.

BACKGROUND OF THE INVENTION

Ferritic-austenitic (duplex) stainless steels are widely used today asconstruction material in a number of industries. Duplex steels are oftendeveloped for especially favorable use in special areas. For example,the duplex steel SAF 2507 (UNS S 32750), which is alloyed with 25% Cr,7% Ni, 4% Mo and 0.3% N and which is described in the Swedish PatentApplication SE-A-453 838, concerned to be especially resistant againstchloric induced corrosion and finds therefore applications asconstruction material if the process solution contains chlorides or ifthe material will be exposed for seawater or chlorine containing coolingwater, for example in heat exchangers.

In U.S. Pat. No. 5 582 656 (SE-A-501 321) duplex steels are described,which contain a maximum of 0.05 weight % C, a maximum of 0.8 weight %Si, 0.3-4 weight % Mn, 28-35 weight % Cr, 3-10 (3-7) weight % Ni,1.0-3.0 (1.0-4.0) weight % Mo, 0.30-0.55 weight % N, a maximum of 1.0weight % Cu, a maximum of 2.0 weight % W, 0.010 weight % S and 0.2weight % Ce, and a balance of Fe together with normally occurringimpurities and additives, and wherein the ferrite content of the steelmakes 30-70 volume %.

A purpose of the present invention is to provide duplex steel for usewithin seawater applications.

As described in SE-A-453 838 the composition of the alloy is not themost important factor to provide such steel. The balance between thedifferent components of the alloy and structural factors is moreimportant. Furthermore it is well-known from this patent that highamounts of, for example, chromium, improve the tendency of precipitationof intermetallic compounds so strong, that problems in manufacturing andin relation with welding could occur. A high amount of nitrogen isdesired in order to stabilize the alloy against precipitation ofintermetallic phases and improvement of the corrosion resistance, but isrestricted by the limited solubility in the melt, which causesprecipitation of chromium nitrides. By these reasons the content ofchromium in this alloy will be restricted to a maximum of 7% and thecontent of nitrogen to 0.25-0.40%.

SUMMARY OF THE INVENTION

Surprisingly, some of the alloys described in U.S. Pat. No. 5,582,656have been found to possess favorable and, in certain cases, particularlygood properties as construction material in the field of seawaterapplications. This result is surprising since these alloys have a highcontent of chromium and high content of nitrogen that is over the upperlimit that taught by to SE-A-453 838 as avoiding precipitation.Especially good properties will be achieved if the PRE-value of thesteel is at least 40.

The invention provides consequently to a steel containing a maximum of0.05 weight % C, a maximum of 0.8 weight % Si, 0.3-4 weight % Mn, 28-35weight % Cr, 3-10 weight % Ni, 1.0-4.0 weight % Mo, 0.2-0.6 weight % N,a maximum of 1.0 weight % Cu, a maximum 2.0 weight % W, a maximum of0.010 weight % S and a maximum of 0.2 weight % Ce, and the balance Fetogether with normally occurring impurities and additives, at which theferritic content makes 30-70 volume % and the PRE-value is at least 40.

None of the steel grades, that are specifically described in U.S. Pat.No. 5,582,656 or SE-A-501 321 provides a PRE-value over 40 in both theferritic and the austenitic phase. Most of the embodiments provide aPRE-value under 40 even calculated on the hole composition.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is schematic illustration of crevice corrosion;

FIG. 2 is a plot of yield point vs. wall thickness necessary towithstand a certain internal pressure;

FIG. 3 is a graphical representation of critical pitting temperature(CPT) for various alloy compositions;

FIG. 4 is a graphical representation of CPT vs. weight % NaCl contentcomparing a steel of the present invention with a conventional steel;

FIG. 5 is a TTT diagram comparing a steel of the present invention withconventional steels;

FIG. 6 is a plot of PRE values vs. temperature, comparing a BCC phaseand a FCC phase of steel according to the present invention;

FIG. 7 is a plot of CPT vs. PRE values;

FIG. 8 is a plot of time to failure vs. stress/tensile strength,comparing a steel of the present invention with conventional steel;

FIG. 9 is a graphical representation of the yield point for articlesformed from alloys of the present invention;

FIG. 10 is a graphical representation of the ultimate strength forarticles formed from alloys of the present invention; and

FIG. 11 is a graphical representation of the elongation for articlesformed from alloys of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Before describing the particular features and aspects of the presentinvention, it should be noted that seawater is not the same all over theworld. For instance, the total amount of dissolved salt can range fromapproximately 8000 mg/l (ppm) in the Baltic Sea to ca 7.5 times thisamount in the Persian Gulf. The total amount of salt that artificialseawater is based on is 35 000 mg/l, which can be considered as atypical amount for seawater. In table 1 the mixture of artificialseawater is shown. The main share of all salt in seawater is NaCl.Often, seawater contains also sand and other solid particles.

The following table shows the mixture of the artificial seawater usedfor testing material suitability for seawater applications.

TABLE 1 Mixture of artificial seawater Concentration % of the totalElement (mg/l) amount of salt Chlorine 18980 55.0 Bromine   65 0.2Sulphate  2649 7.7 Bicarbonate  140 0.4 Fluorine   1 0.0 Boric acid   260.1 Magnesium  1272 3.7 Calcium  400 1.2 Strontium   13 0.0 Potassium 380 1.1 Natrium 10560 30.6 Total 34486 100.0

The primary factors which determine the corrosivity of seawater are:content of chloride, index of pH, temperature, oxidizing ability,biological activity and flow rate. Even impurities in the water canaffect the corrosivity. The temperature of the seawater is stronglyvariable depending upon where one is situated and at what depth thewater is taken. The pH-value of seawater is approximately 8.

A steel according to the invention comprises a maximum of 0.05 weight %C, a maximum of 0.8 weight % Si, 0.3-4 weight % Mn, 28-35 weight % Cr,3-10 weight % Ni, 1.0-4.0 weight % Mo, 0.2-0.6 weight % N, a maximum of1.0 weight % Cu, a maximum of 2.0 weight % W, a maximum of 0.010 weight% S and a maximum of 0.2 weight % Ce.

The PRE-value, i.e. (% Cr)+3.3×(% Mo)+16×(N), should be at least 40 inthe total composition, preferably at least 42 in the total composition.Further, each phase should exhibit a PRE-value over 40, preferably atleast 41.

In U.S. Pat. No. 5,582,656 it is specified that the additional alloyingelements should fulfill the expression % Cr+0.9% Mn+4.5% Mo-12.9% N<35in order to minimize the risk for precipitation of intermetallic phasesduring the production. It has surprisingly been determined that onecould hold the above-mentioned value in the present steel at 35 or more,but still achieve the essential properties which are necessary to beable to use the steel in seawater applications. It is advantageous tohold the above-mentioned value at 35 or more, as it is easier to obtaina higher PRE value. Thus, the steel of the present invention preferablyfulfills the expression % Cr+0.9% Mn+4.5% Mo-12.9% N>35 to obtain asufficiently high PRE value. Preferably, the value of % Cr+0.9% Mn+4.5%Mo-12.9% N is at most 40, and more preferably at most 38.

The preferred content of Mn is 0.3-3.0%, and the content of S issuitably maximum 0.005%. Consequently, a reduced amount of MnS-slag willbe obtained in the material. Those slags easily initiate pitting inseawater-environment, thus it is preferable to keep this type of slag ona low level in a “seawater-steel”.

The content of Mo is preferably 1.5-4.0%. This gives a higherminimum-level for the PRE-value in the steel. However, due to the riskof precipitation of intermetallic phases, the content of Mo should berestricted to a maximum of 3.0%, preferably to a maximum of 2.5%.

For the maintenance of a sufficient high content of Cr in the austeniticphase, and so that the PRE-value should be over 40, the lowest totalcontent of Cr is suitably approximately 29%. In view of the risk ofprecipitation of intermetallic phases the content of Cr shouldpreferably be maximum 33%.

Nitrogen increases the relative content of chromium and molybdenum inthe austenitic phase. Therefore, the content of N should be at least0.30%, but preferably no less than 0.36%. High contents of N could causeformation of voids under welding and therefore the alloy according tothe invention should contain maximum of 0.55% Nitrogen.

The content of Ni is preferably maximum 8%, and the minimum content ispreferably 5%.

An important property for seawater applications is high strength,i.e.—high yield point and high fatigue limit. By providing a materialwith high strength, you can use poorer material (e.g.—thinner wallthickness for tubes), and reduce weight. Often, it is important to keepthe weight of a construction material for seawater applications low,because the construction could often be situated on floating plants,such as boats, oil platforms and so on, thus more available buoyancy totransport goods would be used by heavier materials.

Another important property of material for seawater applications is goodcorrosion resistance in Cl-containing environments. The types ofcorrosion which can easily be initiated in Cl-containing environmentsare pitting corrosion, crevice corrosion and stress corrosion cracking.Pitting and crevice corrosion of the material could be avoided if the“PRE-value” for the same is sufficiently high. The PRE-value is definedas PRE=(% Cr)+3.3×(% Mo)+16×(% N). In order to have a good corrosionresistance in seawater the PRE-value should be higher than 40 for duplexsteel. As apparent from the definition, a high PRE-value could be basedon whether a high content of Cr, Mo or N. It is well-known that a highcontent of Mo gives a less structurally stable material, related to theprecipitation of the sigma phase. It is well-known that a high contentof N gives a more structurally stable material. Therefore it is moresuitable to base the high PRE-value on a high content of N or Cr, ratherthan a high content of Mo.

At risk for crevice corrosion it is also desirable with a high contentof N, because this neutralizes H⁺-ions, which will be formed in thecleft and by that avoid the decreasing pH-value that could make theenvironment worse. The crevice corrosion course is schematically shownin the FIG. 1.

The third type of corrosion, which can appear in Cl-containingenvironments, is stress corrosion cracking. This appears mainly inaustenitic stainless steel and is treacherous, because it can developvery fast. It is well known that duplex steels have very good stresscorrosion cracking resistance because of the advantageous synergisticeffect between the ferritic and the austenitic phase in the material.

Another property, that is important in some cases of seawaterapplications, is the erosion corrosion resistance of the alloy. Theerosion corrosion can be defined as acceleration of the corrosion courseas a consequence of rapidly streaming media, which sometimes can containsolid particles. A strong contributing factor for erosion corrosion isthe turbulent flow in tubes (in contrast to laminar flow). Turbulentflow can be increased by high velocity flow restrictions in the tube,e.g.—valves in the tube, sharp bends, etc.

A last factor to be taken into consideration is of course the price ofthe alloy. For seawater applications it should be desirable with amaterial that has a good corrosion resistance, especially inCl-containing environments, and at the same time has the highestpossible strength.

The steel according to the invention has a very high strength,i.e.—yield point in tension (0.2) ≧650 MPa. In comparison with othertypical steel grades for seawater applications this is considerablehigher (SAF 2507: yield point in tension=550 MPa; 6Mo-steel: yield pointin tension=300 MPa). Due to its high strength, a steel according to thepresent invention can be used in the form of a tube with considerablethinner wall thicknesses than tubes formed from conventional materials.

However, the high strength is not coincident for all steels described inU.S. Pat. No. 5,582,656. For example, there is steel described therein(no. 10) with a yield point in tension of only 471 MPa (Table 1 and 2).However, this steel has a PRE-value at only 35.6 and is, consequently,not within the scope of the present invention.

FIG. 2 shows the effect of the yield point in tension on the wallthickness which is necessary to withstand a certain inner pressure(according to the formula in the Swedish conduit standard 1978, RN78).As evident from FIG. 2, increasing the yield point in tension from 550MPa to 650 MPa allows a reduction of the wall thickness of 15%, and inconnection with this, a reduction of the total tube weight in the range.A corresponding comparison between 300 MPa and 650 MPa reduces about 50%of the weight.

The pitting and crevice corrosion of the presented steel is good. Thisdepends on that the PRE value of the alloy is over 40. More precisely,the PRE value is around 42, which is the same level as for theestablished “seawater steels” SAF 2507 (UNS S 32750) and austeniticstainless steel of the type 6-Mo.

As an acceptance test for such a material, it is common to use tests forthe pitting corrosion, which can be seen as an indicator for theseawater resistance. The most frequent method is to use the modifiedASTM G48A-method, where a material is placed in a solution of 6% ferricchloride, whereafter the temperature is stepped with a 24-hour intervaland the material will be inspected concerning to the pitting corrosionafter every test period. The temperature where pitting corrosion occursis called the critical pitting temperature (CPT). FIG. 3 shows thecritical temperature for specimen of the materials 254 SMO, SAF 250, anda steel according to the invention. From this it can be concluded thatall of these materials have high values for the critical pittingtemperature, and for this reason it is probable that the materials haveequivalent pitting corrosion resistance in seawater.

Corresponding testing in FeCl₃ can be made with applied crevice formers.A steel according to the invention has a critical crevice corrosiontemperature of about 40° C. Even this could be seen as being atapproximately the same level as for the established “seawater steels”.The possibility the development of crevice corrosion after initiationcould even be expected to be on a low level because of the high contentof nitrogen in the alloy.

Another method to determine the material's pitting resistance that isused is an electrochemical test with a steadily applied potential on thematerial. In order to simulate chlorinated seawater, which is a veryaggressive solution, it is tested at 600 mV/SCE. The result of thistesting of a steel according to the invention is shown in FIG. 4. Asapparent, this steel passes 70° C. in this environment, independent ofthe content of NaCl.

As mentioned earlier, the reason for good pitting and crevice corrosionresistance is a high PRE value. A comparison can be made with SAF 2507,which is optimized with respect to the PRE value so that the PRE valueis equal in both phases. This result is obtained by alloying with awell-balanced composition of Cr, Mo and N, and 0.30% N gives balancebetween PRE in the ferritic and austenitic phase, when the content ofchromium is 25% and the content of Mo is 4%. A PRE-value over 40 willthen be achieved.

The steel according to the invention is based on the same presumptions,namely PRE-balance. But, according to the present invention, a highercontent of Cr and a lower content of Mo is chosen, which makes itpossible to alloy a higher content of N. Due to the fact that Mo isconsiderably more detrimental to structural stability than Cr, and alsothat the content of N is higher than in SAF 2507, a higher structuralstability in the steel according to the invention is obtained with asustained PRE-value in the phases (see FIG. 5 for TTT-curve).

FIG. 6 shows the influence of temperature on the PRE value in ferritic(BCC) and austenitic (FCC) phases for the presented steel. PRE balancewill be obtained at about 1080° C., which is the temperature at whichthe material is heat treated and the value of the PRE-value is over 40.

The importance of having a high PRE value in both the ferritic andaustenitic phase is shown in FIG. 7, where the CPT according to ASTMG48A is shown as a function of PRE value for the somewhat weakerferritic phase in some test variants of the steel according to theinvention. A PRE-value over 40 in both phases should therefore beconsidered as fulfilled in connection with a CPT (G48A) of 75° C. forthe final alloy.

As illustrated in FIG. 8, the stress corrosion resistance of the steelaccording to the invention is clearly greater than that of austeniticsteels of type 316. It should be borne in mind that the duplex steelshave a very high strength in absolute figures, which makes thepercentage of the tensile strength which can be effectively utilizedbefore stress corrosion occurs is very high for these steels.

According to the present invention, the impingement attack resistance ofthe steel is very high, with highest reliability, because of the highstrength and the good resistance for duplex steels.

Cu-base alloys are materials that are often used in seawater. However,materials have the big disadvantage of being sensitive to impingementattacks. Other competing materials for seawater applications are Ti- andNi-based alloys. However, these are considerable more expensive than thesteel of the present invention.

The present invention will now be described by reference to thefollowing examples, which are intended to be illustrative rather thanrestrictive.

EXAMPLE

In the following some embodiments of steels according to the inventionwill be described.

In the following Table 2 are compositions shown for five alloysaccording to the invention. These are the examples taken from a largenumber of different alloys which were produced and tested during thedevelopment of the present invention.

TABLE 2 [0055] Alloy C Si Mn Cr Ni Mo N Cu S 1 0.015 0.19 0.91 29.268.00 2.07 0.31 0.025 0.0043 2 0.016 0.16 1.01 28.81 7.48 2.50 0.37 0.0350.0032 3 0.021 0.27 0.90 28.80 6.62 2.20 0.38 0.081 0.0010 4 0.015 0.151.00 29.01 6.66 2.51 0.40 0.037 0.0036 5 0.016 0.16 0.87 30.51 6.20 2.080.44 0.034 0.0042

Extruded bars were formed from alloy no. 1, 2, 4 and 5, the content ofCr, Ni, Mo and N measured in the austenitic and ferritic phases with thehelp of a step by step analysis in a microgroove. The result of thosemeasurements is shown in the following Table 3.

TABLE 3 [0057] Al- loy Phase Cr (%) Ni (%) Mo (%) N (%) 1 Ferritic 32.59± 0.48 5.47 ± 0.18 2.60 ± 0.14 0.00 ± 0.03 Austenitic 27.88 ± 0.31 9.24± 0.20 1.58 ± 0.14 0.62 ± 0.03 2 Ferritic 31.78 ± 0.42 5.27 ± 0.32 3.16± 0.12 0.00 ± 0.02 Austenitic 28.15 ± 0.48 8.48 ± 0.18 1.93 ± 0.08 0.75± 0.03 4 Ferritic 31.58 ± 0.34 4.65 ± 0.13 3.21 ± 0.20 0.01 ± 0.03Austenitic 28.88 ± 0.28 7.45 ± 0.15 1.93 ± 0.10 0.88 ± 0.04 5 Ferritic32.31 ± 0.31 4.58 ± 0.13 2.40 ± 0.11 0.00 ± 0.03 Austenitic 30.16 ± 0.256.99 ± 0.20 1.64 ± 0.13 0.98 ± 0.04

The PRE values, (% Cr)+3.3 (% Mo)+16 (% N), for the alloys were measuredfor each phase, and for the total composition, as shown in the followingTable 4.

TABLE 4 PRE-values for austenitic and ferritic phase in test alloys PREPRE Alloy (total composition) Phase (for different phases 1 41.1Ferritic 41.2 Austenitic 43.0 2 43.0 Ferritic 42.3 Austenitic 46.5 443.7 Ferritic 42.3 Austenitic 49.3 5 44.4 Ferritic 40.2 Austenitic 51.3

As evident from the above, the PRE value is higher than 40 in both theaustenitic and the ferritic phase in all alloys. This is a condition fora good corrosion resistance in seawater.

The PRE-value in the respectively phase could also be calculated by thehelp of the computer-program “Thermo-Calc” based on the composition.This calculation is made for alloy 1 at different temperatures and isillustrated in FIG. 6.

The heat-treatment temperature of about 1080° C. that renders the samePRE value in both phases comes from calculated values. Thus, as would beunderstood by those in the art, is only approximate. Therefore, actualvalues for PRE could deviate a little from equilibrium.

The measured values for the strength of the manufactured tubes of alloyno. 2, 3 and 4 are shown in the diagrams in FIGS. 9-11. It appears thatthese alloys according to the invention have a yield point in tensionover 650 MPa in the product application of a thin-walled tube (<10 mm),which is the general dimension used in seawater applications.

It has surprisingly been demonstrated that the steel according to thepresent invention is well-suited for use in seawater applications. Inthis regard, the steel has a yield point in tension over 650 MPa, whichmeans that about 15% of the tubes weight could be saved compared withSAF 2507 and about 50% compared with 6Mo-steel by reducing the wallthickness. At the same time, the material has a good seawater resistancebecause it has a PRE-value over 40 in both phases and a high stresscorrosion cracking resistance.

While the present invention has been described by reference to theabove-mentioned embodiments, certain modifications and variations willbe evident to those of ordinary skill in the art. Therefore, the presentinvention is to limited only by the scope and spirit of the appendedclaims.

What is claimed is:
 1. A duplex ferritic-austenitic stainless steelalloy provided for seawater applications comprising, in weight %: C maximum 0.05 Si maximum 0.8 Mn 0.3-4   Cr 28-35 Ni  3—10 Mo 1.0-4.0 N0.2-0.6 Cu maximum 1.0 W maximum 2.0 S  maximum 0.010 Ce maximum 0.2

and balance Fe together with normally occurring impurities andadditives, wherein the ferritic content is 30-70 volume %, the alloycomposition has a PRE-value higher than 42, and the PRE-value is atleast 40 in both the ferritic and austenitic phases, where PRE=(%Cr)+3.3×(% Mo)+16×(% N).
 2. The alloy according to claim 1, wherein thePRE-values of the ferritic and austenitic phase are approximately equal.3. The alloy according to claim 2, wherein the alloy is heat-treated atapproximately 1080° C.
 4. The alloy according to claim 1, wherein thecontent of C is maximum 0.03 weight %.
 5. The alloy according to claim1, wherein the content of Si is maximum 0.5 weight %.
 6. The alloyaccording to claim 1, wherein the content of Cr is between 29 and 33weight %.
 7. The alloy according to claim 1, wherein the content of Mois at least 1.5 weight %.
 8. The alloy according to claim 1, wherein thecontent of Mo is maximum 3.0 weight %.
 9. The alloy according to claim1, wherein the content of N is between 0.30 and 0.55 weight %.
 10. Thealloy according to claim 1, wherein the content of N is at least 0.36weight %.
 11. The alloy according to claim 1, wherein the content of Mnis maximum 3 weight %.
 12. The alloy according to claim 1, wherein thecontent of ferrite is between 30 and 55 volume %.
 13. The alloyaccording to claim 1, wherein the content of Cr in the austenitic phaseis at least 25 weight %.
 14. The alloy according to claim 1, wherein thecontent of Cr in the austenitic phase is at least 27 weight %.
 15. Thealloy according to claim 1, in the form of at least one of tubes, bars,heavy castings, forgings, plate, wire or strip.
 16. A method comprising:providing a duplex ferritic-austenitic stainless steel alloy with acomposition comprising, in weight %: C  maximum 0.05 Si maximum 0.8 Mn0.3-4   Cr 28-35 Ni  3—10 Mo 1.0-4.0 N 0.2-0.6 Cu maximum 1.0 W maximum2.0 S  maximum 0.010 Ce maximum 0.2

and a balance Fe together with normally occurring impurities andadditives, wherein the ferritic content is 30-70 volume %, wherein thealloy composition has a PRE-value higher than 42, and the PRE-value isat least 40 in both the ferritic and austenitic phases, where PRE=(%Cr)+3.3×(% Mo)+16×(% N), forming a component with the alloy; andcontacting the component with seawater.
 17. The method according toclaim 16, wherein the compound comprises at least one of tubes, bars,heavy castings, forgings, plates, wire or strip.